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Full text of "Text Book Of Mechanical Engineering"

36                           Malleable Castings.

of cast iron. The iron mould must be painted with a thin coating
of very fine blackwash before casting, and some care must be taken
in the forming of the gates, as, if there should not be sufficient
pressure from the ' head' of metal used, the iron will recoil on
meeting the cold mould, and form a rough casting. Care must
also be used in the case of bushes, to remove the core chill before
the casting cools dovyn firmly upon it. The chills (the name
given to the iron moulds) are usually made of good cast iron,
though, in some rare instances, wrought iron has been used. Some
of the details of chill moulding vary according to different autho-
rities. Some founders purposely rust their chills on being first
made, to assist the blackening in resisting the action of the metal,
it being generally believed that the latter tends to fuse and injure
the chill. Other founders, notably in the case of projectiles,
neither rust them nor use blackening. The chills should be
warmed before casting, in order to expel moisture, and should
have a weight about six times the casting they are to chill, or the
chilling will be .too slow.

v Malleable Castings are obtained by taking the article,
after being cast and cleaned up (this last is very important), and
putting it, along with others, in an annealing furnace, in company
with some substance that will absorb the carbon from the cast
iron. Such substances ?are, oxide of iron in the form of scales
from the rolling-mill, or some other of the metallic oxides, placed
in the furnace in a state of powder. The intensity of the heat,
and the time the casting should remain in the furnace, both
depend on the size of the casting and the amount of malleability
required, the usual rule being to keep it at a white heat for about
a week, adding to this the time required to raise the temperature
and to cool down, i

Fig. 490 shows an annealing furnace with cast iron boxes A A
holding the castings, which are covered with a layer of sand. Of
course, it must be understood that it is only to a short depth
below the surface that the casting becomes converted into
wrought iron, though right through for small articles.
^ Softening.—If a casting is so hard that it cannot be machined,
it may be softened by heating and cooling out in common sand,
or any other bad conductor of heat